In machines used in agriculture, such as for example telescope loaders, wheel loaders, or front-end loaders on tractors, it is known to use a hydraulic suspension system that cushions the boom or the rocker arm in order to achieve an overall improvement of the vehicle suspension and riding comfort, especially when the vehicle is traveling. These types of hydraulic suspension systems use a suitable hydraulic system of valves, a hydraulic cylinder and a hydraulic accumulator. The lifting side of the hydraulic cylinder is connected to the hydraulic accumulator to achieve the required suspension by. In addition, the lowering side of the hydraulic cylinder is connected to a hydraulic tank to avoid cavitation and to enable free movement of the piston rod during the suspension process.
In order to increase safety against a sudden lowering of the boom or rocker arm, these suspension systems have a load holding device to prevent hose breaks. However, in order to lower the hydraulic cylinder it is necessary to close the tank connection of the lowering side of the hydraulic cylinder so that a required pressure can build up in order to open the load holding device. Oil will flow off from the lifting side of the hydraulic cylinder only when the load holding device has been opened.
A hydraulic system for a suspension system of this type is disclosed in EP 1 157 963 A2. The suspension system for the boom of a telescope loader is proposed. The suspension system provides a load holding device for safeguarding against a pressure drop in a hydraulic cylinder and in a hydraulic accumulator. The load holding device essentially comprises a check valve that, in combination with a controllable pressure relief valve, can be bypassed by bringing the pressure relief valve from a normally closed position into an opened position using control pressure lines. In order to avoid limiting the functionality of the suspension system, or to avoid hindering an exchange of hydraulic fluid between the hydraulic accumulator and the hydraulic cylinder, the load holding device is situated at the supply side before the hydraulic cylinder and the hydraulic accumulator. A disadvantage of this system is that it provides only one load holding device for safeguarding pressure-loaded hydraulic components. A hose break occurring between the hydraulic cylinder and the hydraulic accumulator would result in the boom falling downward, and is thus not safeguarded by the load holding device.
Therefore, a need exists to create a hydraulic system of the type described above that provides a separate safeguard for the hydraulic cylinder and the hydraulic accumulator, while at the same time providing a suspension function.